Troubleshooting Steam Surface Condensers

Troubleshooting Steam Surface Condensers
Troubleshooting Steam Surface
Condensers
HTS Forum
12 September 2012
Steam Condenser Troubleshooting
What is a steam surface condenser?
Troubleshooting tools
Is there a problem?
Fault diagnosis
Locating air leaks
Condenser monitoring
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Troubleshooting Steam Surface Condensers
What is a Surface Condenser?
Steam
Work
Turbine
Condenser
CW
What Does it Do?
Condenses the exhaust steam
→ Rejects the latent heat at the lowest
practical temperature
→ Condenses under vacuum
By creating vacuum, the condenser
increases the efficiency of the turbine
The condenser also performs some other
important functions
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Troubleshooting Steam Surface Condensers
Condenser Temperature Profile
Temperature, °C
Temperature approach
Tsat
CW out
∆T
CW in
Condenser duty
Duty, kW
Q = U A ∆T , if U and A are large, ∆T can be small → vacuum
Typical Condensers
Round condenser
(up to about 3m diameter)
Rectangular condenser
(for larger units)
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Troubleshooting Steam Surface Condensers
Condenser Vacuum Pumps
Condensers are provided with vacuum
pumps/ejectors
The condenser creates the vacuum,
NOT the vacuum pump
Vacuum pumps are there to extract air
and pump it up to atmospheric pressure
A startup ejector (“hogger”) is provided to
create a rough vacuum in order to start
the turbine
Condenser Venting
LP Steam
Air Pump
Shell / Hood
Cooling
Water
Tube Bundle
Cooling Water
Waterbox
Tubesheet
Condensate
Hotwell
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Troubleshooting Steam Surface Condensers
Troubleshooting Tools
Is There a Problem?
Plant operator is typically concerned
about turbine exhaust pressure
First task is to establish whether the
exhaust pressure is higher than expected
Two issues:
– The plant measurement may not be accurate
– The turbine exhaust pressure will naturally
vary with plant load and CW temperature
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Troubleshooting Steam Surface Condensers
Case Study
6 x 660MW generating units, turbine exhaust pressure (kPa) by unit:
1
2
3
4
5
6
6.85
7.01
3.23
6.98
6.89
6.81
CW temperature = 25°C
Comments?
Condenser Performance Curve
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Troubleshooting Steam Surface Condensers
Identifying Faults
Three most common causes of
condenser performance problems:
– Air accumulation in condenser
– Cooling water fouling
– Reduced cooling water flow
Unfortunately, all have the same effect
on condenser performance:
– Increase in turbine exhaust pressure
Condenser Temperature Profile
Temperature, °C
Temperature approach
Tsat
CW out
∆T
CW in
Condenser duty
Duty, kW
What is the response to the three most common faults?
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Troubleshooting Steam Surface Condensers
Effect of Air in Steam Space
No air
Air “blanket”
Steam condenses easily Steam must diffuse
Minimal sub-cooling
Sub-cooling
The presence of air reduces the heat transfer rate
Fault Diagnosis
To differentiate between the various
causes of high turbine exhaust pressure,
examine temperatures, not pressure:
– CW flow reduction is indicated by increase in
CW temperature rise
– Fouling is indicated by increase in
temperature approach (Tsat – CW out)
– Air accumulation is indicated by increase in
temperature approach and increased ∆T
between Tsat and condensate temperature
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Troubleshooting Steam Surface Condensers
Confirming and Fixing Faults
Reduction in cooling water flow is caused
by fouling or a pump fault
Fouling can be confirmed by inspection
(most condensers have divided waterboxes)
Air accumulation is a more complex issue
For air to accumulate:
– Vent rate too low (vacuum pump fault)
– Ingress rate too high (air leak)
Vacuum Pump Faults
Generally rare, although original
specification may be deficient
For LRVPs check seal water temperature
For steam ejectors check motive steam
pressure (too high or too low)
Detailed troubleshooting outside scope of
this presentation
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Troubleshooting Steam Surface Condensers
Air Leaks
Some in-leakage is expected (hence the
vacuum pump)
Vacuum pump is usually over-specified
(especially if to HEI Standard)
Once excess capacity is used, any
additional in-leakage will cause turbine
exhaust pressure to rise
Vacuum boundary must be maintained
Air Leakage Points
Turbine burst disc
Expansion
joint
Turbine shaft seals
Turbine
Vacuum
breaker
PI
Instrument
connections
Manway
seals
Condenser
Valve stems
CEP seals
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Troubleshooting Steam Surface Condensers
Air Leak Detection Methods
Many methods exist
Online methods:
–
–
–
–
Feathers / Smoke / Candles
Ultrasonics
Helium testing
Halogen testing
Offline method – water fill test
Online testing with tracer gases is the most
successful approach
Traditional Methods?
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Troubleshooting Steam Surface Condensers
Tracer Gas Testing
Air pump
Detector
Tracer Gas Testing - Tips
Always check the detector is working before
you start
Avoid false indications:
– Helium: work from top to bottom
– Halogen: work from bottom to top
– Time the response
Consider testing under low load conditions:
– The vacuum boundary is more extensive at low
loads
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Troubleshooting Steam Surface Condensers
Case Study
200MW electrical generation unit in Ireland
High turbine exhaust pressure on startup
after outage, close to turbine trip point
Outage scope had included:
– Condenser cleaning
– LP turbine rotor removal
Heat balance indicates full CW flow
Hotwell temperature inconclusive
Case Study - Actions
Evidence suggests air leak
High probability of leak at turbine shaft
seals
Helium testing ordered to confirm diagnosis:
–
–
–
–
Warranty issue
Shutdown required to repair
Which shaft seal is leaking?
Other leaks?
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Troubleshooting Steam Surface Condensers
Case Study - Results
Leakage at both turbine shaft seals
confirmed
Additional leakage also found at:
– Turbine bursting disc
– Cracked weld on pipe
– Plugged connection on condenser shell
Resolution of non-shaft leaks moved turbine
pressure away from trip point without
shutdown
Leak Locations
Turbine burst disc
Cracked weld
Shaft seal
Plugged connection
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Troubleshooting Steam Surface Condensers
Condenser Monitoring
Critical situations can be avoided by regular
monitoring
Monitoring pressure not very useful
Monitor key temperature differences:
– (CW out – CW in)
– (Tsat – CW out)
– (Tsat – Thotwell)
Perform vacuum decay testing
– Isolate vacuum pump and measure rate of
pressure increase
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Page 15